Start port forwards on the phone, listed in a sheet off its terminal

The Android head skipped a host's forwards because it had nowhere to show
what was listening. Its terminal bar now draws a ⇄ for a tab that has
forwards, carrying the listening count in green and turning the danger
colour when one failed, and it raises a sheet over the same rows the
desktop sidebar lists: notation, route, state, failure reason and a tap
to switch each off or on. MainWindowViewModel.StartForwards no longer
returns early on Android.

The sheet is shell state like the connect sheet: it collapses the
renderer while up, is lowered by leaving the terminal, and the back
gesture closes it first.

Forwards ride the shell's keep-alive rather than holding the foreground
service themselves, since a forward cannot outlive its shell. They only
change the notification, which now counts ports forwarded beside shells
connected, fed by a new ForwardsChanged event and ListeningForwardCount.

The phone still has no editor for forwards; the More screen card, README
and design-import-gaps now say they run here and are edited on the
desktop.

src/DodoSSH.Client.Shell/ViewModels/TerminalTabViewModel.cs

@@ -342,6 +342,20 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
     /// <summary>How many of <see cref="Forwards"/> are listening now.</summary>
     internal int ListeningForwardCount => Forwards.Count(forward => forward.IsListening);
 
+    /// <summary>Whether any of <see cref="Forwards"/> is listening now.</summary>
+    internal bool HasListeningForward => ListeningForwardCount > 0;
+
+    /// <summary>Whether this tab carries any forwards at all, running or not.</summary>
+    /// <remarks>The phone's terminal bar draws its FORWARDS button only for a tab that has some.</remarks>
+    internal bool HasForwards => Forwards.Count > 0;
+
+    /// <summary>Whether any of <see cref="Forwards"/> could not be started.</summary>
+    /// <remarks>
+    /// The phone's list is a sheet, not a sidebar that is always drawn, so a refusal there would be invisible
+    /// until somebody happened to open it. This is what lets the button that opens it say so first.
+    /// </remarks>
+    internal bool HasFailedForward => Forwards.Any(forward => forward.IsFailed);
+
     /// <summary>
     /// The status bar's forward count, or null when nothing is listening and there is nothing to say.
     /// </summary>
@@ -394,6 +408,9 @@ internal sealed partial class TerminalTabViewModel : ObservableObject
     {
         OnPropertyChanged(nameof(ListeningForwardCount));
         OnPropertyChanged(nameof(ForwardsSummary));
+        OnPropertyChanged(nameof(HasListeningForward));
+        OnPropertyChanged(nameof(HasForwards));
+        OnPropertyChanged(nameof(HasFailedForward));
     }
 
     /// <inheritdoc cref="TerminalTabState" />